Gas sampling is regularly performed to determine the amount of various target compounds in an area. The samples may be taken over a short period of time indicating “instantaneous” exposure of personnel to the compounds or taken over an extended period of time indicating the average exposure of the personnel for that period of time. Samples are typically captured in a sample bag with the use of a sampling device comprising a sampling pump.
There are basically two conventional sampling systems, a direct sampling system and an indirect sampling system. A typical direct sampling system is shown in FIG. 1A. As shown in FIG. 1A, a conventional direct sampling apparatus or device 61 comprises a sampling pump 60 that draws a gas to be sampled from the surrounding environment and discharges the sampled gas through the tubing 65 into a sample bag 62. The sample bag 62 comprises an inlet 66 with a tubing connection. The inlet 66 may further be attached to a valve that may be opened during the sampling process and closed to retain the sample in the sample bag 62. The sample bag 62 may then be removed from the sampling device 61 and sent to a laboratory for analysis. In a direct sampling apparatus, the inlet of the sampling pump 60 is in fluid communication with the area to be sampled and the outlet of the pump 60 is in fluid communication with the inner volume of the sample bag 62. As such, the gas to be sampled flows through the sampling pump 60 and tubing 65. In such a direct sampling system, the gas contacts the internal components of the sampling pump 60 and the inner wall of the tubing 65, this contact may result in contamination of the sampled gas or loss of a portion of the sample as it attaches to or reacts with the material in the pump or with the walls of the tubing resulting in a sample that represents less than the actual concentration of the contaminant. The internal components of the pump and inner wall of the tubing may still comprise a residue of previously sampled gas or may be contaminated from a cleaning or maintenance procedures. To eliminate the chance of contamination of the sampled gas and/or loss of a portion of the components of the sampled by contact with the sampling pump or other components of the direct sampling apparatus, indirect sampling apparatuses may be used.
A typical indirect sampling apparatus is shown in FIG. 1B. As shown in FIG. 1B, a conventional indirect sampling apparatus 63 also comprises a sampling pump 60. However, the sampling pump 60 in an indirect sampling method draws air from inside a hermetically sealed box 64 (sometimes referred to as a “lung box”) to create a vacuum. A lung box 64 is a rigid walled hermetically sealed box with a connector for the pump 60 inlet and a connector 65 to provide fluid communication between the sample bag 62 with the exterior area to be sampled. A sample bag 62 within the lung box 64 expands due to the vacuum and thus draws gas from the area to be sampled through tubing 65 into sample bag 62. The sample bag 62 in an indirect sampling system also comprises an inlet 66 with a tubing connection and a valve that may be opened during the sampling process and closed to retain the sample in the sample bag 62. The sample bag 62 may be removed from the lung box 64 and sent to a laboratory for analysis. As the pump draws air out of the lung box 64, the walls of the gas-sampling bag 62 are pulled apart by the resultant vacuum thus increasing the inner volume of the sample bag 62 and providing driving force for the ambient gas to be sampled to fill the sample bag 62. The indirect sampling apparatus 63 may be more bulky than a direct sampling apparatus 61 but provides a lower risk of contamination, cross-contamination of samples and/or loss of a portion of the contaminant. Drawbacks for both of the conventional direct and indirect sampling apparatuses include the necessity of carrying and storing bulky equipment, charging the pump batteries, maintaining and calibrating the pump regularly and calibrating the pump by trained personnel before and after use of the pump for time weighted average (TWA) samples, and to establish a clean stationary sampling place. Further, in certain applications such as, but not limited to, chemical, petrochemical, petroleum, and natural gas facilities, the electronic pumps of direct and indirect sampling apparatuses must be certified as intrinsically safe to ensure the electronic pump does not create a spark sufficient to cause an explosion or a fire.
The high prices of both direct and indirect sampling apparatuses and the ancillary equipment affect the overall cost of the sampling. These sampling apparatuses require that the sampling pump be well calibrated and can pump consistently particularly when performing a sampling process through an extended period.
A major drawback of conventional sample bags with thin monolayer walls is their comparably short storage life of a sample retained within the sample bag. Samples may be stored reliably in such sample bags typically from 24 to 48 hrs. or up to 72 hrs. for some specific gases sampled. After this time, the sample gas has changed composition by off-gassing or sorption of components on the interior walls of the sample bag. The stability of a sample and the recovery (%) of target gases is limited by the composition of the inner layer; nevertheless such thin monolayer walls have been developed and are widely used for construction of sample bags.
An advance in sample bag technology was the introduction of multilayer walls for sampling bags. An embodiment of multilayer wall has a sandwich construction comprising at least three layers. The multilayer walls have at least one outer layer comprised of a chemically resistive low permeability material. The multilayer walls have an intermediate layer comprising a thin aluminum layer, wherein the aluminum is provided as a foil or by vacuum metallization process. This aluminum layer slows diffusion through the multilayer wall. The inner layer is a polyolefin layer (typically polyethylene that may be chosen from various density polyethylenes) that serves as a thermal sealing layer. The polyolefins have comparably good barrier properties, but also a tendency to emit (outgas or off-gas) some monomers, plasticizers, and other additives, thereby contaminating the sample. The inner polyolefin layer also has a tendency to adsorb some organics. These drawbacks limit their use mainly to sampling of inorganic gases or to short term sample retention uses. Both single layer and multiple layer walls have inherent permeability and/or sorption which limit the recovery of target gases in the sample to 80-85% and also limit the stability of the sample between 24 to 72 hrs.
Conventional sample bags comprising multilayer materials were adapted from bags developed to meet the requirements and regulations of the food packaging industry. Regulations for bags for food packaging uses allow a certain amount of organics, typically monomers, plasticizers, and/or other additives, to outgas from the plastic. The conventional bag inner layer outgasses certain quantities of compounds that may be acceptable for food storage applications. However, these quantities of outgassed components have the potential to contaminate the sampled gas in a sample. The outermost external layer of conventional multilayer sample bags comprises material that is highly resistive to absorption and diffusion such as polyester and nylon, but these materials have relatively high melting points and therefore are not as desirable as thermal sealing layers. Thus, the outer layer, though it may not be easily sealed together, prevents loss of sample integrity by diffusion through the wall, but the inner layer, which can be easily sealed, results in loss of sample integrity by off-gassing.
A more sophisticated sampling apparatus includes a SUMMA Canister 70 as shown in FIG. 1C. A SUMMA canister is a stainless steel vessel which has specially passivated internal surfaces using a “Summa” passivation process. A Summa passivation process combines an electro-polishing step with chemical deactivation to produce a surface that is chemically inert. Due to the passivation of the surface, chemical compounds are not absorbed on the surface and samples retained in a SUMMA canister are stable for a longer period than a sample retained in a conventional sample bag. To draw a sample into the canister, the pressure within the SUMMA canister 70 is reduced to vacuum of approximately twenty-eight inches mercury to remove substantially all the gas in the canister 70. The residual gas is typically uncontaminated air or ballast such as nitrogen or other inert carrier gas. The SUMMA sampling apparatus comprises a special flow regulator that may be calibrated to achieve predetermined sampling time of, for example, 15 minutes, 30 minutes, 1 hour, 2 hours or up to 24 hrs. The sampling process is typically finished when the pressure in the SUMMA canister has risen to about 2 inches of mercury vacuum; therefore, the canister is still under vacuum even after sampling. To facilitate withdrawal of the sample from the canister for analysis or other use, the SUMMA canister 70 must subsequently be pressurized with an inert carrier gas or filtered calibration grade clean air. The inert carrier gas or filtered calibration grade clean air raises the pressure within the SUMMA canister without contaminating the sample. However, adding gas in the pressurization process and the original gas in the canister after reducing the vacuum to 28 inches results in a dilution of the concentration of the target gases in the sample.
After pressurization, an aliquot volumetric analysis sample of the diluted gas is withdrawn for analysis. Each step including vacuuming, sampling, and pressurizing of the Summa canister is monitored by use of a pressure gauge and the accuracy of monitoring each step depends on the accuracy and reliability of the pressure gauge to calculate volumes of gas in the canister. In many cases, the pressure gauges used with SUMMA canisters do not have accuracy necessary or are not calibrated precisely enough for extremely accurate determination of the dilution ratio between the gas actually sample and the residual gas in the container and the gas added during the pressurization process. Therefore, there is an inherent systematic error in the gas concentration calculations and target gas analytical determination. As such, the accuracy of overall method is compromised from the many steps and is prone to errors.
The disadvantages of using a SUMMA canister sampling apparatus include the initial high costs of the canister, the high cleaning cost of the interior of the canister, high maintenance costs of the canister and peripheral equipment, the high cost of purchasing and maintaining a special cleaning system in specialized labs, the high cost of special gauges and expensive flow controllers, the necessity of a precise flow calibration for each extended sampling period, the necessity of constant observation during a sampling period to end the sampling process so the pressure does not exceed the limit of 2 inches of mercury vacuum, the necessity of accurately pressurizing the SUMMA canister with a carrier gas or filtered calibration grade clean air, the high cost of the inert carrier gas cylinder and cylinder demurrage or the cost of creating the filtered grade clean air, the necessity of performing additional calculations after chemical analysis, and the necessity to know the initial sampling conditions including temperature, barometric pressure, and altitude above sea level.
In accordance with drawbacks discussed above, there is a need for a sampling systems, devices and methods which will eliminate at least a portion of the drawbacks of the conventional sampling methods but still provide the benefits.
There is a need for a sample bag that limits contamination be off-gassing and still prevents loss of sample integrity by diffusion through the sample bag walls or adsorption of chemical compounds on the sample bag walls.